This invention relates to a control system for controlling a frequency of an AC power supplied to a primary winding of an induction motor including a shortcircuited secondary conductor, for example, a squirrel-cage motor.
In conventional induction motor control systems the variable frequency power supply device, for example, the inverter or cyclo-converter has supplied an electric power to an induction motor with a shortcircuited secondary conductor, typically, to a squirrel-cage motor so that the electric power has a frequency as determined by the algebraic sum of an output from a command slip frequency generator responsive to a command torque and an output from a speed sensor for the motor. A voltage for supplying the electric motor is proportional to the absolute value of the frequency thus determined. Such control systems have more or less improved the characteristics of the generated torque but the slip of the motor does not fast respond to the required torque. In addition, an air gap flux on the motor has been changed and long time has gone until this change in gap flux is settled.
In order to solve those problems, the motor has been provided with an air gap-magnetic sensor thereby to control the AC power supply to the motor in response to the sensed position of the gap flux or a phase of an internal electromotive force. This measure has resulted in a complicated, expensive structure.
It is an object of the present invention to provide a new and improved induction motor control system for controlling a frequency of electrical energy supplied to a primary winding of an induction motor including a shortcircuited secondary conductor, having a fast response and a simple circuit configuration without the necessity of providing a gap flux sensor and subordinate controls.